Abstract

Spatially uniform ground motion is an assumption that has often been made for
structural analysis of arch dams. However, it has been recognized for many years
that the ground motion in a canyon during an earthquake is amplified at the top
of the canyon relative to the base. Pacoima Dam has been strongly shaken by the
1971 San Fernando earthquake and the 1994 Northridge earthquake. The acceleration
records from both of these events demonstrate the spatial nonuniformity of the ground
motion, but the amount and quality of the data made it difficult to study in detail. An
opportunity to do so arose on January 13, 2001, when a relatively small magnitude 4.3
earthquake was recorded by an upgraded accelerometer array at Pacoima Dam.
Frequency-dependent topographic amplification is apparent at locations along
both abutments at 80% height of the dam relative to the base. Also, the ground
motion is delayed at the abutment locations compared to the base. The delays are
consistent with seismic waves traveling upward along the canyon, and the waves appear
to be dispersive since the delays are frequency-dependent. Both of these effects
are quantified in this thesis by several approaches that involve varying degrees of
approximation. A method for generating nonuniform ground motion from a single
3-component ground motion specified for one location in the canyon, e.g., at the base,
is developed using transfer functions that quantify the amplification and phase delay.
The method is demonstrated for the 2001 earthquake and the Northridge earthquake
with several variations in the transfer functions.
The 2001 earthquake records were also used for system identification. These results
do not agree with results from a forced vibration experiment, which indicate a
stiffer system. The earthquake must induce nonlinear vibrations, even though the excitation is quite small. This observation has implications for applications of structural
health monitoring.
The generated nonuniform ground motions are supplied as input to a finite element
model. The results indicate that the method for generating nonuniform input
produces ground motion that yields reasonable modeled responses, but there is some
evidence that the time delays may be larger for stronger ground motion. Comparisons
of the responses from ground motions generated with various implementations
of amplification and time delays were made. For modeling purposes, accuracy of
the amplification appears to be more important than the delays, which can be dealt
with using a simpler approximation. The nonuniform input produces a response that
is substantially different than the response produced by uniform input. The major
difference is that while the pseudostatic response is a rigid body motion for uniform
input, it causes deformation of the dam, mostly close to the abutments, for
nonuniform input. In order to refine the proposed method for generating nonuniform
ground motion, more data is required from Pacoima Dam and other structures with
instrumentation coverage along the abutments.